Tomato lines segregation for resistance to cucumber mosaic virus

15 lines were bred by interspecific hybridisation and I1 to I6 generation o f three of them were tested for resistance to CMV. In spite of the selectio n by CMV resistance in the progenies the number of the resistant plants did not always increase. The progenies having 100 % symptomless plants for two or more consecutive years were not selected in the studied lines. A large spectrum of variations in the percentage of symptomless plants in the proge nies per year and the presence of disease and symptomless parts in one plan t were established. These results are possibly due to the complex mechanism of inheritance of CMV resistance as well as to the influence of environmen tal factors on the expression of the resistance observed.